The practice of selection for certain production characteristics in livestock dates from prehistoric time. Traits are considered important because of the economic values which contribute to the usefulness of animals as a source of meat protein.
Animals processing superior inheritance must be recognized by the breeder in order to make genetic improvement. Until recent times, selection in swine has been based largely upon visual appraisal. It is more effective, however, to take precise measurements of economically important traits so that selection can be employed to achieve progress in these areas.
Among those traits considered important are reproductive efficiency and body composition. Early efforts involved the measurement of fat and meat content of the live animal by use of mechanical devices and ultrasound. Selection for reduction of backfat thickness in the live animal constitutes an important genetic factor for improvement of the lean-to-fat ratio. It is possible to make animal breeding progress by use of such information on animals in the animal breeding scheme.
Other efforts have been employed to determine factors relating to reproductive efficiency. Such efforts include biopsy of vaginal genitalia as well as Doppler methods. In the case of Doppler methods, a degree of difficulty rested with operator technique in the required ability to differentiate between "noise" levels of the environment and those related to fetal blood flow. Biopsy techniques were laborious and required additional laboratory facilities and were not practical to utilize on a large sale. Use of ultrasound with a transmitting and receiving unit, however, makes it possible to identify the presence of certain characteristics rather than their movement.